The present invention relates to the general field of the medical imaging.
1. FIELD OF INVENTION
More particularly, the invention applies to the generation of mechanical waves in a viscoelastic medium, with such mechanical waves being likely to be imaged in order to determine the properties of the viscoelastic medium.
The present invention therefore relates more precisely to the field of elastography.
2. DESCRIPTION OF RELATED ART
This medical imaging technique maps the mechanical properties of a viscoelastic medium and quantifies the rheology of the viscoelastic medium. According to this technique, a mechanical stimulus is generated and causes displacement of the tissues. The spatiotemporal response of the tissue to this mechanical excitation is then measured. The spatiotemporal response is advantageously measured using an imaging modality, for example by echography or magnetic resonance, etc.
Once the movement resulting from the mechanical excitation is known, it is possible to determine the mechanical properties of the medium.
In transitory elastography, the mechanical excitation consists of a short mechanical pulse or a low number of pulses created either on the surface of the body, or inside the tissue itself.
The quality of transitory elastography images depends crucially on the amplitude of the shifts possible to be generated by exciting mechanical stimulation.
It is evident in transitory elastography by external stress, that the amplitude in shift is limited only by the maximum surface vibration which can be induced at contact of the medium without damaging it. The resulting shifts in tissue easily have amplitudes of the order of 100 μm.
In this way, and in general, shifts resulting from mechanical excitation must be sufficient to be measurable with the fewest errors, at the same time being limited to avoid any harmful effect in the medium, especially in the case of biological tissue.
The generated power is therefore satisfactory, though it is known that use of external stress creates technical problems, such as the space requirement of the device necessary for this stress, synchronisation of the mechanical excitation with the imaging, localisation of the mechanical excitation, optimisation of the amplitude of the wave in the zones of interest in depth, etc.
There is also transitory elastography where the mechanical stress of the medium observed is created by an acoustic radiation force. This radiation force is obtained by focusing an ultrasound beam inside the medium. Focusing the beam can take place here in a single zone of the medium or successively in a plurality of zones of the medium.
The focal spot, on which the ultrasound beam converges, is then moved at a speed greater than the propagation speed of the elastic waves to generate an elastic shift wave of maximum amplitude of the order of 10 to 100 μm.
This shift wave propagates in the medium. Measuring the propagation properties of the wave, observed by echography, MRI or some other imaging modality, determines mechanical variables characteristic of the tissues investigated. It is possible to determine, inter alia, a shearing module or even viscosity, etc.
The shift engendered by the acoustic radiation force is connected to the energy deposited in the tissue, and the amplitude of the mechanical wave generated is therefore limited by the maximum acoustic power which can be sent in the observed medium without thermally or mechanically altering the tissue.
The ultrasound solution offers simple handling, reproducibility of the manner in which the stress is generated, assurance as to synchronisation of excitation with imaging and assurance as to localisation of the excitation, but suffers from a lack of power.